1. Field of the Invention
The present invention is generally related to methods and apparatuses for frequency translation and frequency selectivity.
2. Related Art
FIG. 1 is a block diagram of an example conventional receiver 112. FIG. 2 is a flowchart representing the operation of the receiver 112.
In step 206, a band-select filter 102 filters an RF (radio frequency) spectrum 114. An example RF spectrum 114 is shown in FIG. 4A. The RF spectrum 114 includes signal components at frequencies f.sub.1, f.sub.2, f.sub.3, and f.sub.4. Assume, for purposes of example, that the receiver 112 is configured to receive signals at frequency f.sub.3.
Typically, the band-select filter 102 is a wide-band filter. The characteristics of the band-select filter 102 are generally illustrated in FIG. 4B. The band-select filter 102 has a center frequency f.sub.c, and a band-select bandwidth 402. In the example shown in FIG. 1, where the receiver 112 is receiving an RF spectrum 114, the center frequency f.sub.c of the band-select filter 102 is within the RF range. For example, the center frequency f.sub.c may be 900 MHZ. Depending on the application, the band-select bandwidth 402 may be as much as 50 MHz, or greater. In the example where the center frequency f.sub.c is 900 MHZ and the band-select bandwidth 402 is 50 MHZ, the passband (i.e., the band of frequencies that pass through a filter with little loss, relative to frequencies outside of the band) of the band-select filter 102 is 875 Mhz to 925 MHz. According to these specifications, the quality factor of the band-select filter 102, or Q, is equal to 18 (as described further below, Q is equal to the center frequency divided by the bandwidth, or 900 MHz.div.50 Mhz in this example). This Q factor is typical for a band-pass filter operating at RF. In fact, generally, high Q factors at high frequencies are difficult to realize using conventional filter techniques, and have at best limited tuning capabilities.
The band-select filter 102 in step 206 operates to filter out signals outside its passband. For example purposes, assume that f.sub.1 and f.sub.4 are outside the passband of the band-select filter 102, and f.sub.2 and f.sub.3 are inside the passband of the band-select filter 102 (this is the case in the example of FIGS. 4A and 4B). Accordingly, in this example, the band-select filter 102 operates to filter out the signal components at frequencies f.sub.1 and f.sub.4. The band-select filter 102 passes the signal components at frequencies f.sub.2 and f.sub.3. The result of the operation of the band-select filter 102 is shown in FIG. 4C.
In steps 208 and 210, the signal output by the band-select filter 102 (herein called the band-select filtered spectrum 408 for reference purposes) is processed by a low-noise amplifier (LNA) 104 and a mixer 106. The LNA 104 operates to amplify the band-select filtered spectrum 408, and the mixer 106 operates to down-convert the band-select filtered spectrum 408 in a well known manner.
Both the LNA 104 and the mixer 106 have limited dynamic ranges over which their operation is linear. Outside of these ranges, the LNA 104 and the mixer 106 exhibit non-linear operation. The broader the band select filter 102 (i.e., the wider the pass band), the more energy is able to reach the LNA 104 and the mixer 106. Consequently, the broader the band select filter 102, the greater the chance that the respective dynamic ranges of the LNA 104 and the mixer 106 will be exceeded. For purposes of example, assume that the signal component 420 at frequency f.sub.3 combined with the undesired signal component 421 at frequency f.sub.2 exceed the linear ranges of the LNA 104 and the mixer 106 (this is a common practical example).
When operating on a signal that is outside their linear ranges (i.e., when operating in a non-linear manner), the LNA 104 and/or the mixer 106 generate spurious signal components. In the given example, when operating on the signal components 420 and 421, the LNA 104 and/or the mixer 106 generate spurious signal components 404. See FIG. 4D. Some of these spurious components 404 may coincide and interfere with signals at desired frequencies. For example, as noted above, the receiver 112 is tuned to receive signals at frequency f.sub.3 (in the example of FIGS. 4A-4G, frequency f.sub.7 corresponds to f.sub.3 after downconversion; similarly, frequency f.sub.6 corresponds to f.sub.2 after downconversion).
In the process of operating on the signal components 420 and 421, the LNA 104 and/or the mixer 106 generate a spurious signal component 404C at frequency f.sub.7. This spurious component 404C coincides with the desired signal component 420 at frequency f.sub.7. This spurious component 404C interferes with the desired signal component 420.
In step 212, a channel-select filter 108 filters the signal generated by the LNA 104 and the mixer 106 (this signal is herein called the processed spectrum 410 for reference purposes). The characteristics of the channel-select filter 108 are generally shown in FIG. 4E. The channel-select filter 108 has a center frequency at frequency f.sub.7 and a channel-select bandwidth 406. The center frequency f.sub.7 of the channel select filter 108 is at a lower frequency than the center frequency of the band select filter 102. For example, the center frequency f.sub.7 of the channel select filter 108 may be 10 MHZ. Depending on the application, the channel-select bandwidth 406 may be, for example, 50 KHz. According to these specifications, the quality factor of the channel-select filter 108, or Q, is equal to 200 (as indicated above, and described further below, Q is equal to the center frequency divided by the bandwidth, or 10 MHz.div.50 KHz in this example). This Q factor is typical for a narrowband band-pass filter operating at IF (intermediate frequency). As this example illustrates, it is possible to realize higher Q factors at lower frequencies using conventional filter techniques.
As shown in FIG. 4F, the effect of the channel-select filter 108 in step 212 is to filter-out the signal component at frequency f.sub.6 and spurious components 404A, 404B, and 404D, but to pass any signals at frequency f.sub.7. Both the desired signal component 420 and the spurious component 404C exist at frequency f.sub.7, and are within the passband of the channel-select filter 108. Thus, both the desired signal component 420 and the spurious component 404C are passed by the channel-select filter 108.
In step 214, an amplifier 110 amplifies the signal output from the channel-select filter 108 (this signal is called the channel select filtered signal 412 for reference purposes). The channel select filtered signal 412 includes both the desired signal component 420 and the spurious component 404C. Consequently, the amplifier 110 amplifies both the desired signal component 420 and the spurious component 404C.
In other words, once the spurious component 404C is generated, it follows the desired signal component 420 in all downstream processing.
As noted above, the spurious signal component 404C may make it difficult if not impossible to properly receive the desired signal component 420. Accordingly, because the receiver 112 utilized a wide-band, band-select filter 102 prior to amplification and frequency translation by non-linear components (i.e., by the LNA 104 and the mixer 106, respectively), the receiver 112 suffers from potentially degraded performance. The potential for signal interference as described above limits the receiver 112's applicability.